In 1996, biochemist Michael Behe introduced the notion of “irreducible complexity,” arguing that some biomolecular structures could not have evolved because their functionality requires interacting parts, the removal of any one of which renders the entire apparatus defective. This claim excited creationists and remains a central plank of the “intelligent design” movement, despite being rightly rejected by a U.S. federal judge in 2005 in Kitzmiller v. Dover Area School District. In Darwin Devolves, Behe continues his quixotic efforts to overturn modern evolutionary theory.


Exaptation—when evolution retools structures for new purposes, as happened with birds’ feathers—can be mistaken for irreducible complexity.



In the grand scheme of evolution, mutations serve only to break structures and degrade functions, Behe argues. He allows that mutation and natural selection can explain species- and genus-level diversification, but only through the degradation of genes. Something else, he insists, is required for meaningful innovation. Here, Behe invokes a “purposeful design” by an “intelligent agent.”

There are indeed many examples of loss-of-function mutations that are advantageous, but Behe is selective in his examples. He dedicates the better part of chapter 7 to discussing a 65,000-generation Escherichia coli experiment, emphasizing the many mutations that arose that degraded function—an expected mode of adaptation to a simple laboratory environment, by the way—while dismissing improved functions and deriding one new one as a “sideshow” (1). (Full disclosure: The findings in question were published by coauthor Richard Lenski.)

Behe also ignores the fact that some of his prior arguments have been dismantled (2). He includes a lengthy appendix that argues that the blood-clotting cascade is irreducibly complex, for example, but fails to mention Kenneth Miller’s simple, elegant scheme for its stepwise evolution (3) or the fact that a progenitor fibrinogen gene has been discovered in echinoderms (4).

Behe doubles down on his claim that the evolution of chloroquine resistance in malaria by random mutations is exceedingly unlikely because at least two mutations are required, neither of which is beneficial without the other. His calculations have already been refuted (5), and it has long been known that neutral and even deleterious mutations can provide stepping stones to future adaptations. Indeed, a 2014 study, unmentioned by Behe, reported discovery of two genetic paths through which malaria has evolved chloroquine resistance through multiple steps (6).

Missing from Behe’s discussion is any mention of exaptation, the process by which nature retools structures for new function and possibly the most common mechanism that leads to the false impression of irreducible complexity. Some Sphingomonas bacteria, for example, have evolved the ability to digest a wood preservative, pentachlorophenol, by recruiting two unrelated biochemical pathways (7). Neither pathway can do that job alone, yet there they are together. The feathers of birds, gas bladders of fish, and ossicles of mammals have similar exaptive origins.

Exaptation also challenges Behe’s notion of “devolution” by showing that loss of one function can lead to gain of another. The evolutionary ancestors of whales lost their ability to walk on land as their front limbs evolved into flippers, for example, but flippers proved advantageous in the long run.

Behe is skeptical that gene duplication followed by random mutation and selection can contribute to evolutionary innovation. Yet there is overwhelming evidence that this underlies trichromatic vision in primates (8), olfaction in mammals (9), and developmental innovations in all metazoans through the diversification of HOX genes (10). And in 2012, Andersson et al. showed that new functions can rapidly evolve in a suitable environment (11). Behe acknowledges none of these studies, declaring an absence of evidence for the role of duplications in innovation.

Behe asserts that new functions only arise through “purposeful design” of new genetic information, a claim that cannot be tested. By contrast, modern evolutionary theory provides a coherent set of processes—mutation, recombination, drift, and selection—that can be observed in the laboratory and modeled mathematically and are consistent with the fossil record and comparative genomics.

Ultimately, Darwin Devolves fails to challenge modern evolutionary science because, once again, Behe does not fully engage with it. He misrepresents theory and avoids evidence that challenges him.


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